dereferencing var|lent uint(8|16|32) yields wrong value in VM

[alaviss]

Example

proc foo(values: openArray[uint16]) =
  for value in values.items:
    doAssert value == 40000u16:
      "Got: " & $value

foo([40000u16])

Actual Output

$ nim r --backend:vm test.nim
stack trace: (most recent call last)
test.nim(1, 2) NimMain
test.nim(6, 4) test
test.nim(3, 14) foo
assertions.nim(38, 26) failedAssertImpl
assertions.nim(28, 11) raiseAssert
fatal.nim(50, 5) sysFatal
fatal.nim(50, 5) Error: unhandled exception: test.nim(3, 14) `value == 40000'u16` Got: 18446744073709526080 [AssertionDefect]

Additional Information

• Values between 0 - 32767 (high(int16)) does not exhibit this issue.
• This only happens if the array was accessed via items.

[zerbina]

Thanks for filing the issue!

The problem is not directly related to openArray and uint16, but instead affects all lent uint(8|16|32) and var uint(8|16|32) dereferences, with the reason being that vmgen doesn't emit the necessary narrowing instructions.

A more minimized reproducer:

proc f(x: (uint8,)): lent uint8 = x[0]

echo f((128'u8,)) # echoes 0xFFFFFFFF_FFFFFF80 (as a non-hex number)

[saem]

Fixed in #1409